The determining of digestion parameters in liquid samples plays a role, for example, in process measurements technology or in industrial measurements technology, especially in the field of water and waste water treatment and/or in water and waste water analysis. Important examples of digestion parameters include chemical oxygen demand (COD), total carbon content and total nitrogen content (Ntot).
Chemical oxygen demand is the oxygen equivalent amount of a chemical compound, usually a strong oxidizing agent, which is consumed by the oxidizable constituents contained in a certain volume of a liquid sample under the reaction conditions of a prescribed method. Serving as oxidizing agent, in such case, is frequently potassium dichromate. The COD value is, besides the total nitrogen content, an important parameter for classifying the degree of pollution in the case of river water and in waste water and clarification plants, especially those containing organic impurities.
In known methods for determining such digestion parameters, first of all, the liquid sample is mixed with a digestion agent in a digestion container, most often as one embodied as a cuvette. This reaction mixture is heated, in given cases, under pressure, for a predetermined time in the digestion container. The substances to be detected, on whose concentration the given digestion parameter depends, are, in such case, dissolved by chemical reaction with the digestion agent. Depending on the type of digestion parameter to be detected, either the consumption of the digestion means or a reaction with one or more additional reagents added to the reaction mixture brings about a change in the chemical and/or physical properties, e.g. the extinction, respectively absorption, of the liquid sample. This change can be detected, for example, by means of a suitable electrochemical transducer or, e.g. in the case of a change of extinction, respectively absorption, by means of a photometric, measuring transducer. The current value of the digestion parameter can be ascertained based on a measurement signal provided by the measuring transducer.
In the case of most methods for determining chemical oxygen demand, for example, a sample is treated with a known excess of an oxidizing agent and then the consumption of the oxidizing agent ascertained, for example, through back titration of the not consumed remainder. Photometrically ascertaining the consumption of oxidizing agent represents another option, e.g. when potassium dichromate serves as the oxidizing agent. The amount of consumed oxidizing agent is converted into equivalent oxygen amount.
Known from the state of the art is a series of analytical devices for determining digestion parameters according to such methods. Described in German patent application DE 103 60 066 A1, for example, is an analytical device for photometrically determining the chemical oxygen demand of a liquid sample, wherein a reaction mixture in a cuvette of the liquid sample and potassium dichromate as oxidizing agent is heated under pressure-tight closure for a digestion time at a temperature above the atmospheric boiling temperature of the reaction mixture. At the same time, the extinction of the reaction mixture in the cuvette as the digestion progresses is determined at at least one fixed wavelength, wherein the change of the extinction serves as a measure for the concentration change and therewith for the consumption of the oxidizing agent.
Furthermore, described in International Published Application, WO 002005064328 A1 is a method for metering a liquid, wherein the liquid in a metering chamber with light barriers is dosed only by means of a piston pump. In the case of this embodiment of the dosing, metering and transport unit, the following problems occur. Due to the long hose length for sample taking in the sample container, only a limited sucking distance and sucking height is possible by means of this arrangement. The sample liquid can only be sucked in by a negative pressure supplied by the piston pump. If the volume to be sucked in is, due to the hose length of the sample liquid transport line, greater than the suction volume of the piston pump, predetermined volumes can be sucked in only by multiple actuations of the piston pump and therewith increasing the negative pressure only by multiple operation of the piston pump. This greatly increases the time consumed for pumping the liquid samples, since piston pumps move only relatively slowly, and, moreover, the piston pump experiences increased wear from the greater mechanical loading and the higher negative pressure.